Gren, Anders

Abstract [en]

The Skellefte district is one of the most important mining districts in Sweden and hosts over 80 massive sulfide deposits. The Blylodtorpet area is located in the eastern part of the district about 8 km west of Boliden. This work was set up with the aim to classify the rock types in the area, map geological structures and from this see if the geological environment is favorable for ore forming processes or not. The work contains a field study of the about 4 km2 large Blylodtorpet area, structures visible in outcrops were been measured, two drill cores were logged and 18 samples taken from outcrops and drill core were analyzed and interpreted. Thin sections from all 18 samples analyzed were also investigated with a microscope The most dominant rock type of the area is dacitic mass flow units, they have a clast size of mainly gravel-sand but also block-sand and sand-silt have been found. Andesitic dykes are also present in some outcrops they have a steep dip usually around 80o and often contains sulfides. Ultramafic rock where identified in two outcrops, there is a difference in the geochemical composition between this two ultramafic rocks,one of them have a more primitive pattern in the spider diagram and have higher content of Mg,Cr and Ni. Andesitic rock and clastic sediments are also found in the area and in one outcrop of clastic sediments the bedding planes has a very flat dip and an electromagnetic anomaly is also present over this outcrop. The geochemical data classifies the area as a part of an island arc of a continental margin, and the rocks follow a calc alkaline trend in discrimination plots. The area was folded at least twice with axial planes striking NE–SW and ESE- WNW respectively. Parts of the area have very flat dipping bedding planes and on some of them a way up determination were made. In one of the drill cores a quartz-feldspar porphyry was identified at two different levels. The most interesting ore minerals found in thin sections are sphalerite and arsenopyrite, but the most common sulfide is pyrrhotite. The quartzfeldspar porphyry and the various sulfides are all signs that can indicate the existence of an interesting, hidden mineralization.